1. Field of the Invention:
This invention relates generally to RF power sources, and more particularly to a high frequency voltage-controlled oscillator signal generator whose output signal frequency is determined by the amplitude of an externally applied tuning signal, and which is suitable for use in narrow-band FMCW applications.
2. Discussion of the Prior Art:
In millimeter wave or Ka band systems, such as radar and communications, there is a need for a signal generator or driver whose frequency can be varied with an applied low frequency or DC bias. The Singh U.S. Pat. No. 4,574,256, which is assigned to applicants' assignee, describes such a device and is somewhat closely related to the present invention. Like the present invention, it utilizes microstrip technology along with a Gunn diode and varactor to create a VCO for use in the Ka band. The VCO described in the aforereferenced Singh patent is ideally suited for broadband FMCW transmitting applications in that it is capable of providing a tuning bandwidth well in excess of two GHz when constructed with a hyperabrupt varactor diode. The tuning sensitivity of such a VCO is in excess of 200 MHz/V. Moreover, by utilizing a hyperabrupt varactor, a very linear frequency versus tuning voltage characteristic is achieved.
Where it is desired to provide a narrow-band FMCW driver, it is necessary that the tuning sensitivities be on the order of only 1 MHz/V. Using the circuit of the Singh patent, a frequency change of .+-.1 MHz results when only a tuning voltage change of .+-. 5 mV is applied. Such a small voltage is less than the typical electrical noise level existing in DC power supplies and, as such, the circuit of the Singh patent is found to be ill-suited for narrow-band FMCW applications.
It is accordingly a principal object of the invention to provide an improved millimeter wave microstrip VCO having an adjustable tuning sensitivity means incorporated therein and which is suitable for narrow-band FMCW applications. By utilizing a hyper-abrupt varactor, a very linear frequency versus tuning voltage characteristic is retained.